Cambridge IGCSE Physics · Syllabus 0625 · Thermal Physics
Evaporation
What is Evaporation?
Evaporation is the escape of the more energetic particles from the surface of a liquid, changing it to a gas; it occurs only at the surface, can happen at any temperature below the boiling point, and cools the liquid left behind because the particles remaining are the slower ones.
This definition is part of the Thermal Physics chapter in Cambridge IGCSE Physics.
Evaporation in context
Topic 2 of Cambridge IGCSE Physics 0625 contains 45 printed statements: 24 Core, taught to all candidates, and 21 Supplement, taught in addition to Core for candidates following the Extended route. The Core statements cover the kinetic particle model and states of matter, Brownian motion as evidence for that model, gas pressure and the qualitative gas relationships, the kelvin scale, thermal expansion and its applications, internal energy, changes of state and evaporation, and thermal energy transfer by conduction, convection and infrared radiation with basic everyday applications. The Supplement statements add inter-particle forces, force per unit area in gases, \(pV = \text{constant}\), the particle explanation of expansion, average kinetic energy and specific heat capacity with its two experiments, the boiling–evaporation comparison and evaporative cooling of an object, the mechanisms and limits of conduction, the radiation balance including the Earth's temperature, emitter and absorber experiments, and complex multi-route applications.
Heating a substance normally increases the average separation of its particles, so the substance expands; Extended candidates also compare how much each state expands. A rise in temperature increases an object's internal energy. When a pure substance melts or boils, the energy supplied separates the particles against the forces between them instead of speeding them up, so the temperature stays constant during the change; condensation and solidification are the reverse, with particles coming closer together and energy being given out. Evaporation is different again: it happens only at the surface, at any temperature below the boiling point, and it cools the liquid left behind.
Common mistakes with Evaporation
- 6. “Energy is needed to make a gas condense.” Core 2.2.3.3 Why it is wrong: it reverses the direction of the energy flow. Condensation and solidification bring particles closer together, which releases energy. Corrected model: moving towards the gas end — melting, boiling, evaporation — takes energy in. Moving the other way — condensation, solidification — gives energy out. Exam-safe: “As the gas condenses, the particles slow down and come close enough for the forces between them to hold them together, and energy is given out to the surroundings.”
- 7. “Evaporation occurs throughout a liquid.” Core 2.2.3.4 Why it is wrong: a particle deep inside is surrounded on all sides by neighbours attracting it and has no free direction in which to escape. Corrected model: evaporation is a surface process; only surface particles can leave the liquid. Exam-safe: “Only particles at the surface can escape, so evaporation takes place at the surface of the liquid.”
- 8. “The coldest particles escape during evaporation.” Core 2.2.3.5 Why it is wrong: slow particles do not have enough energy to overcome the attractive forces. If the slowest left, the liquid would warm up, not cool. Corrected model: the most energetic surface particles escape, so the ones remaining are slower. Exam-safe: “The most energetic particles escape from the surface, so the particles remaining are slower and the liquid cools.”
Examiner tips on Evaporation
- Command-word cue “State the change of state” wants one word: melting, solidification, boiling, condensation or evaporation. Writing “it turns into a liquid” describes the outcome but does not name the process, so it does not answer a question that asked for the name.
- Five phrases that keep a Topic 2 answer precise “…so the average separation of the particles increases…” (expansion) “…collisions with unit area of the wall become more frequent…” (pressure) “…the fluid expands, so its density decreases, so it rises…” (convection) “…the energy separates the particles, increasing their potential energy rather than their kinetic energy…” (change of state) “…the most energetic particles escape from the surface…” (evaporation) “…it transfers energy away at the same rate as it receives it…” (constant temperature, Extended)